WHAT DOES THE GEOMETRY OF CONTRACTIONAL GROWTH STRATA TELL US ABOUT FOLDING?

It is commonly felt that the geometry of contractional growth strata indicates the folding mechanism, and therefore fault-related fold type. The common theory is that fold limbs that grow by kink-band migration (such as in fault-bend folds) develop triangles of growth strata concordant with the pregrowth beds, whereas, fold limbs that grow by limb rotation (such as in many detachment folds) develop growth strata that fan in dip, becoming progressively steeper with time. Differences from this theory observed in natural examples are generally ascribed to nontectonic processes such as compaction or primary sedimentary dip. We show that any folding mechanism that causes lateral changes in vertical velocity of initially horizontal growth strata will cause the beds to fan in dip with continued deformation whether the mechanism is limb rotation, kink-band migration or heterogeneous strain. With more complex, and realistic models, than have previously been considered it is often not possible to distinguish folding mechanism based on the simple criterion of growth triangles vs. fanning of dip. A more complete analysis of the velocity fields implied by growth strata is required to truly elucidate the folding mechanism; however some general rules appear to exist. For example, growth strata that display nonbisecting synclinal axial surfaces, regardless of whether or not the strata fan, suggest at least partial growth by kink-band migration. Natural examples of fault-related folds fit this more general theory.